1. Field of the Invention
This invention relates to heating systems for use with a reaction chamber of chemical vapor deposition equipment and, more particularly, to a heating system for use with a horizontal gas flow reaction chamber having a rotatable susceptor for supporting a single substrate.
2. Discussion of the Related Art:
In the art of manufacturing semiconductor devices, it has long been a practice to employ chemical vapor deposition equipment for depositing various materials on substrates at high temperatures. Chemical vapor deposition equipment includes a reaction chamber configured to contain and control the flow of a reactant and carrier gas therethrough. A base, referred to as a susceptor, is located in the reaction chamber for supporting the substrates upon which the material is to be deposited; the reaction chamber, the susceptor and the substrate are heated to a desired reaction temperature.
A type of prior art structure discloses a slablike susceptor fixedly supported in a horizontal reaction chamber to support a multiplicity of substrates or wafers. A first radiant heat source is located above the reaction chamber and includes a plurality of elongated parallel radiant heating elements lying along horizontal axes above the reaction chamber A second radiant heat source is located below the reaction chamber and includes a plurality of elongated parallel radiant heating elements lying along horizontal axes transverse to the axis of the first radiant heat source. Thermal sensors are located in the susceptor to monitor the temperatures in various regions of the susceptor. The thermal sensors produce signals indicative of the temperatures sensed in their respective regions. Appropriate circuits and controls, responsive to the sensors, adjust the temperatures of the first and second radiant heat sources in the various regions to produce a substantially flat temperature profile in the susceptor.
While simultaneous deposition of materials on a multiplicity of substrates is desirable from a manufacturing standpoint, it has some drawbacks from a quality standpoint. The first problem associated with multi-substrate processing relates to the carrier gas which contains the atoms of the deposition materials. As the reactant gas flows over the surfaces of the substrate and the susceptor, deposition of the materials results in changes in the concentration of the deposition materials in the carrier gas. Consequently, as the reactant gas flows across the susceptor, any individual substrate and multiplicity of substrates, different rates of growth of the deposition of material may occur. A second problem is that of temperature control, which control is critical at the elevated temperatures needed for proper deposition. It is difficult, if not impossible, to control the temperature within the critical tolerances at all the desired locations within the relatively large reaction chambers. This results in different deposition layer thicknesses from one substrate to another and can even produce varying thickness of deposition between individual substrates. Still another problem is contamination which can result from various factors such as: the handling techniques used to load and unload the substrates; the reactant and carrier gases, and the reaction chamber itself. The reactant gas not only deposits the material on the substrate but also on the walls of the reaction chamber. In the relatively large reaction chambers required for multisubstrate processing, the unwanted wall deposits can be inadvertently incorporated into the layers being deposited on the substrates.
These problems and other factors all contribute to significant problems as the semiconductor devices produced from the substrates and the uses to which they are put become more sophisticated. Consequently, many changes and improvements have been made in the equipment that is used to process simultaneously a multiplicity of substrates. For example, some equipment manufacturers are now using automated loading and unloading devices to eliminate contamination resulting from human handling. However, there are practical limits which many feel will ultimately make the simultaneous multisubstrate processing techniques unacceptable. One of the limitations is that of adapting the equipment to handle larger diameter substrates. The economics attendant large diameter substrates are beneficial. Increasing the size of the substrate results in problems with regard to temperature differentials across the substrate, decreasing concentrations of the deposition materials across the substrate and the like. Therefore, steps have been taken by some equipment manufacturers to make single substrate processing equipment significantly simpler in controlling the various factors involved in chemical vapor deposition. Single substrate processing is more desirable than multisubstrate equipment for large substrates, i.e. six to eight inches and more in diameter. One important consideration is the cost at risk when processing one instead of several substrates; that is, if something goes wrong, the monetary loss is far less with one substrate than it is with a plurality of substrates.